All metal components suffer from a degree of residual stress build up caused by, for example, mechanical or thermal loading. This loading may be repetitive in nature such as is experienced, for example, by railway lines, valve springs and engine blocks, or may be a one time event, such as the thermal loading reulting during welding. The presence of stress affects the physical properties of a metal component and can result in stress fatigue and failure of the component.
Techniques for the relief of residual stress build up are known. Generally the known techniques involve heating the component to above its recrystalisation temperature, thereby allowing the crystalline structure to reformulate at a lower energy level. Such techniques are costly, and the process can take days to complete as a long cooling down period is usually required. This means a component will be out of commission for at least a few days and possibly weeks. For larger components the problem is particularly difficult to deal with, as the availability of a sufficiently large oven can be difficult to locate.
Methods available for measuring residual stress build up are not in widespread useage. Current practise is generally simply to subject a metal component to thermal stress relieving techniques regardless of the actual need. Therefore a significant wastage of resources arises in heat treating the those metal components not requiring stress relief.
A related problem arise with elongate components such as railway lines and pipelines. A railway track is generally laid in such a way as to be under a neutral load condition at a predetermined temperature. When the track is above the neutral temperature the railway line as a whole is placed under compression as the sections expand. At excessive levels this can result in the track buckling. At temperatures lower than the neutral temperature the track lengths exist in tension. At some point if the tensile forces are high enough, ie the temperature is low enough, the sections of track can snap.
Because of the outcome of the snapping of sections of track is not as much of a safety hazard as buckling of track the neutral temperature is typically set above the average summertime temperature. In New Zealand the neutral temperature is set at around 30 degrees celsius.
Railway lines undergo considerable thermal cycling. They are also subjected to significant mechanical loading as trains ride over the rails. This can result in plastic deformation of the rails, that is, the rails stretch. When that happens the neutral temperature of the rails drops, and thus the risk of track buckling on hot days increases. It is generally accepted that railway lines need to be reset, or restretched, ie re-laid under tension in order to reset the neutral temperature, every ten years or so to minimise the risk of buckling. It is, at present, a costly and time consuming exercise to uplift a railway line and relay it. Significant resources are wasted on restretching sections of track which do not require any stretching because until now a convenient a method of testing a line to determine whether it requires restretching has not been available.
It is an object of the present invention to provide a method of and an apparatus for testing a metal component which overcomes at least some of the abovementioned problems, or which at least provides the public with a useful choice.
It is a further object of the present invention to provide a method of and an apparatus for relieving the build up of residual stress in metal components which overcomes at least some of the abovementioned problems, or which at least provides the public with a useful choice.